1. Field
The field of the invention is retorting apparatus and methods for removal of mercury from ores and mercury contaminated materials.
2. State of the Art
Mercury is a toxic material to many living organisms, including humans, and its presence in ores or in other materials constitutes a hazard when such materials are processed. Thus extraction of valuable metals such as silver and gold from ores is often complicated by the presence of mercury. The mercury is generally in elemental form, or alloy form (amalgam), or as a sulfide and is easily vaporized. Since the refining of such ores often involves the application of heat, the mercury tends to change to highly toxic vapor.
Removal of the mercury by retorting involves the application of heat to the ore or other mercury bearing material to cause this vaporization. The ore is heated in a reduced pressure environment to accelerate the vaporization. However, although mercury is relatively easily vaporized by contact with high temperature gases such as air, it is difficult to create enough actual liquid mercury contact with the hot gases because both mercury vapor and liquid inherently resist migration through the ores, even though the ores are finely ground. Because of this characteristic, prior art methods of ridding ores of mercury by heating under vacuum is a slow process. The mercury bearing material must be spread very thinly upon trays or pans within the heating oven before the mercury can be thoroughly removed. The toxic nature of the mercury vapor adds to the difficulty of retorting using such pans. Access to the pans for manual stirring in the high temperature vacuum environment is virtually impossible. The oven must be shut down, and allowed to thoroughly cool before the trays can be removed and replaced with fresh unretorted material.
There is therefore a strong need for a method of retorting mercury bearing materials using elevated temperatures and lowered pressure which is not limited by the requirements of prior art retorts for treating only very thin layers of the material.